Martin Wienold

A compact, continuous-wave terahertz source based on a quantum-cascade laser and a miniature cryocooler

We report on the development of a compact, easy-to-use terahertz radiation source, which combines a quantum-cascade laser (QCL) operating at 3.1 THz with a compact, low-input-power Stirling cooler. The QCL, which is based on a two-miniband design, has been developed for high output and low electrical pump power. The amount of generated heat complies with the nominal cooling capacity of the Stirling cooler of 7 W at 65 K with 240 W of electrical input power. Special care has been taken to achieve a good thermal coupling between the QCL and the cold finger of the cooler. The whole system weighs less than 15 kg including the cooler and power supplies. The maximum output power is 8 mW at 3.1 THz. With an appropriate optical beam shaping, the emission profile of the laser is fundamental Gaussian. The applicability of the system is demonstrated by imaging and molecular-spectroscopy experiments.

4.7-THz Local Oscillator for the GREAT Heterodyne Spectrometer on SOFIA

The design and the performance of a 4.7-THz local oscillator (LO) for the GREAT (German REceiver for Astronomy at Terahertz frequencies) heterodyne spectrometer on SOFIA, the Stratospheric Observatory for Infrared Astronomy, are presented. The LO is based on a quantum-cascade laser, which is mounted in a compact mechanical cryocooler. The LO provides up to 150 μW output power into a nearly Gaussian shaped beam. It covers the frequency range from approximately +2 to -4 GHz around the fine structure line of neutral atomic oxygen, OI, at 4.7448 THz. The LO has been successfully operated on SOFIA during six observation flights in May 2014 and January 2015.

High-temperature, continuous-wave operation of terahertz quantum-cascade lasers with metal-metal waveguides and third-order distributed feedback.

Currently, different competing waveguide and resonator concepts exist for terahertz quantum-cascade lasers (THz QCLs). We examine the continuous-wave (cw) performance of THz QCLs with single-plasmon (SP) and metal-metal (MM) waveguides fabricated from the same wafer. While SP QCLs are superior in terms of output power, the maximum operating temperature for MM QCLs is typically much higher. For SP QCLs, we observed cw operation up to 73 K as compared to 129 K for narrow (≤ 15 μm) MM QCLs. In the latter case, single-mode operation and a narrow beam profile were achieved by applying third-order distributed-feedback gratings and contact pads which are optically insulated from the intended resonators. We present a quantitative analytic model for the beam profile, which is based on experimentally accessible parameters.

Fast 2-D and 3-D Terahertz Imaging With a Quantum-Cascade Laser and a Scanning Mirror

A terahertz imaging system based on a quantum-cascade laser (QCL), a fast scanning mirror, and a sensitive Ge:Ga detector is demonstrated. Transmission images are obtained by scanning the beam of the QCL across an object. Images with a diameter of approximately 40 mm and a signal-to-noise ratio of up to 28 dB were obtained within 1.1 s. The system was also used to obtain three-dimensional images of objects in an ellipsoidal volume with axes of approximately 40 mm by computed tomography within 87 s.

Frequency modulation spectroscopy with a THz quantum-cascade laser

We report on a terahertz spectrometer for high-resolution molecular spectroscopy based on a quantum-cascade laser. High-frequency modulation (up to 50 MHz) of the laser driving current produces a simultaneous modulation of the frequency and amplitude of the laser output. The modulation generates sidebands, which are symmetrically positioned with respect to the laser carrier frequency. The molecular transition is probed by scanning the sidebands across it. In this way, the absorption and the dispersion caused by the molecular transition are measured. The signals are modeled by taking into account the simultaneous modulation of the frequency and amplitude of the laser emission. This allows for the determination of the strength of the frequency as well as amplitude modulation of the laser and of molecular parameters such as pressure broadening.

Nonlinear transport in quantum-cascade lasers: The role of electric-field domain formation for the laser characteristics

Many nonlinear transport phenomena in semiconductor superlattices are related to the occurrence of negative differential resistance (NDR) and the formation of electric-field domains (EFDs). Common phenomena, which have been intensively investigated, include discontinuities in the current-voltage (I-V) characteristics and current self oscillations. 1‐3 Although quantum-cascade lasers (QCLs) are based on significantly more complex heterostructures, similar effects are known to exist. EFDs have been observed below threshold in mid-infrared quantum-cascade structures 4 and also in quantum-cascade structures designed for THz emission. 5 In this work, we report on terahertz QCLs, for which the presence of EFDs affects the continuous wave (cw) I-V and output power characteristics above threshold, resulting in the observation of unusual discontinuities in the experimental laser characteristics. We further propose a method to simulate the QCL characteristics in the presence of NDR and EFDs. QCL simulations are usually performed under the assumption of an equal voltage drop in each period, which implies charge neutrality in each period. 6‐8 Such conditions, labeled periodic voltage drop (PVD), have the advantage of a much reduced numerical complexity, but exclude the formation of EFDs, which requires charge accumulation and depletion across multiple periods. We will show how the equations describing the nonlinear transport in weakly coupled superlattices can be used to simulate the transport and output power characteristics of QCLs in the presence of EFDs.

Evidence for frequency comb emission from a Fabry-Pérot terahertz quantum-cascade laser.

We report on a broad-band terahertz quantum-cascade laser (QCL) with a long Fabry-Pérot ridge cavity, for which the tuning range of the individual laser modes exceeds the mode spacing. While a spectral range of approximately 60 GHz (2 cm(-1)) is continuously covered by current and temperature tuning, the total emission range spans more than 270 GHz (9 cm(-1)). Within certain operating ranges, we found evidence for stable frequency comb operation of the QCL. An experimental technique is presented to characterize frequency comb operation, which is based on the self-mixing effect.

Low-threshold terahertz quantum-cascade lasers based on GaAs/Al0.25Ga0.75As heterostructures

We investigated the influence of the barrier composition on the performance of GaAs-based terahertz (THz) quantum-cascade lasers (QCLs). Based on a nine-quantum-well active region design for 3–4 THz emission, QCLs with an Al content of x=0.15 and x=0.25 in the AlxGa1−xAs barriers are compared. We found a significantly reduced threshold current density for QCLs with x=0.25 as compared to QCLs with x=0.15, which is due to a weaker coupling of the subband states. The maximum output power and operating temperature of such lasers are reduced due to the onset of negative differential resistance.

Analysis of the slope efficiency for terahertz quantum-cascade lasers

The slope efficiency is, in addition to the threshold current density and operating voltage, a decisive target value for the operation of quantum-cascade lasers (QCLs) in order to achieve an optimal total efficiency for the conversion of electrical input power into optical output power. We analyze the light-current characteristics for a set of similar, high-power, bound-to-continuum terahertz QCLs. The calculated internal slope efficiency shows a clear dependence on the height and thickness of the barriers. In contrast to the simulations, we found experimentally a significant difference in the threshold current densities and slope efficiencies for QCLs with nominally identical layer structures, which we mainly attribute to different line broadening.

High-spectral-resolution terahertz imaging with a quantum-cascade laser

We report on a high-spectral-resolution terahertz imaging system operating with a multi-mode quantum-cascade laser (QCL), a fast scanning mirror, and a sensitive Ge:Ga detector. By tuning the frequency of the QCL, several spectra can be recorded in 1.5 s during the scan through a gas cell filled with methanol (CH3OH). These experiments yield information about the local absorption and the linewidth. Measurements with a faster frame rate of up to 3 Hz allow for the dynamic observation of CH3OH gas leaking from a terahertz-transparent tube into the evacuated cell. In addition to the relative absorption, the local pressure is mapped by exploiting the effect of pressure broadening.